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A new method of producing malicious PDF files has been discovered by the avast! Virus Lab team. The new method is more than a specific, patchable vulnerability; it is a trick that enables the makers of malicious PDF files to slide them past almost all AV scanners.

Overall, PDF specifications allow many different filters (such as ASCII85Decode, RunLengthDecode, ASCIIHexDecode, FlateDecode, …) to be used on raw data. In addition, there is no limit on the number of the filters used for a single data entry. Anyone can create valid PDF files where the data uses, for example, five different filters or five layers of the same filter. All of these features are based on extremely liberal specifications, a fact which allows bad guys to utilize malicious files in a way that does not allow antivirus scanners access to the real payload.

The new trick is based just on one filter, so it doesn’t sound exciting, does it? So what’s the reason for posting this blog post?

The filter used to encrypt text data is meant to be used only for black and white images. And apart from avast!, probably no other AV scanner is currently able to decode the payload because no other AV can detect those PDF files.